1. Field of the Invention
The present invention relates to a variable capacitance circuit which is suitable for fine adjustment of a frequency-amplitude characteristic or a frequency-phase characteristic of a negative-feedback operational amplifier.
2. Description of the Related Art
In some cases, in the negative-feedback amplifier employing the operational amplifier (hereinafter referred to as OP-amplifier), adjustment of a frequency-amplitude characteristic or a frequency-phase characteristic is necessary. In such a case, generally, variable capacitance circuit is employed. A method of adjustment is that a feedback resistor or an input resistor of the negative-feedback amplifier is connected to a trimmer capacitor of minor capacitance in parallel, followed by adjusting the electrostatic capacity. Such adjustment method is applied to, for instance, a notch filter and so forth employing the above-described negative-feedback amplifier in order to obtain a deep notch characteristic with high xe2x80x9cQxe2x80x9d. In such a case, exceedingly fine adjustment is required. Further, in this case, it is desirable that a temperature characteristic of a variable adjustment circuit itself for correction employed for the sake of this adjustment is made to meet with a temperature characteristic, which is required for the whole other circuits.
Consequently, a variable capacitance circuit for characteristic correction, which is employed for achieving an object of such adjustment, is required to possess function capable of establishing the electrostatic capacitance as well as the temperature coefficient in connection with respective necessary values. However, a trimmer capacitor with such function is not developed yet. Of course, it is possible to select a trimmer capacitor with necessary temperature characteristic from among many trimmer capacitors. However, since such selection requires enormous times, it is not realistic.
An object of the present invention is to provide a variable capacitance circuit which has two functions that electrostatic capacitance of the circuit can be varied such as the trimmer capacitor and further it is possible to establish the electrostatic capacitance against its necessary predetermined temperature coefficient that is temperature compensation function.
In order to attain the above-described object, a variable capacitance circuit of the present invention is provided with a first circuit for generating an output voltage which leads an input voltage by a phase angle of 90xc2x0, a second circuit for performing in phase amplification to the output voltage, followed by taking out an output current corresponding to this amplified output voltage, and variable means for varying the output current of the second circuit.
Also, in the variable capacitance circuit of the present invention, it is suitable that a temperature compensation temperature-sensitive circuit is combined with the first or the second circuit.
Further, in the variable capacitance circuit of the present invention, it is suitable that the second circuit includes an operational amplifier and a temperature transducer is made to combine with an input circuit of the operational amplifier, or the first circuit is configured by a differentiating circuit consisting of a capacitor and a resistance circuit and the temperature transducer is made to combine with the resistance circuit.
Furthermore, in the present invention, it is suitable that the second circuit is an amplifier of a current output type. While it is suitable that the second circuit includes an amplifier of a voltage output type and variable resistor connected to output of the amplifier.
The trimmer capacitor has a function of varying magnitude of electrostatic capacity. This function is that an output current which leads an applied input voltage by a phase angle of 90xc2x0 changes magnitude thereof. Consequently, with respect to this function, in the first place, a phase of applied input voltage is made to advance by a phase angle of 90xc2x0, then, this voltage with the phase angle of 90xc2x0 advanced is subjected to in phase amplification by an amplifier before this amplified output voltage is taken through an output resistance. Since the output current flowing through the resistance is the same phase as a current flowing through the above described electrostatic capacity, if an amplification degree of the above-described amplifier and/or the output resistance are made to vary, magnitude of the output current can be varied. Therefore, the above described function can be realized. Further, this amplification degree and/or the output resistance can be varied, and if a temperature transducer such as thermistor and so forth is combined with this circuit, sensitivity can be adjusted. This enables the circuit to have a necessary temperature coefficient.